The invention relates to a control system for a vacuum pump used for removing liquid and a method of controlling said pump. The vacuum pump arrangement utilized in the invention may be used e.g. in drying machines for paper, paperboard and pulp as well as e.g. in various suction filters and suction washers in a pulp mill. In general, the pump control arrangement according to the invention may be used in all applications where gas, liquid or their mixture is withdrawn through a perforated or wire-like surface.
The use of various vacuum systems as means for facilitating filtration and precipitation is generally known in addition to the paper and wood processing industry, also in e.g. the mining and fertilizing industry. Vacuum systems are utilized in connection with both various wire filters and wire precipitators, into which category the drying machines for paper, paperboard and pulp can be defined, and various vacuum disc or vacuum drum filters. A well-known vacuum arrangement is the use of a barometric leg, by which method the weight of the filtrate flowing into a so-called barometric leg, an essentially vertical tube, creates the necessary vacuum. Because it is practically impossible to regulate the vacuum created by the barometric leg, various vacuum pumps have been employed, the most common of which is the so-called water ring pump. The water ring pump creates typically a constant-volume flow. The suction capacity of the water ring pump may be regulated for example by introducing additional air into the pump through a regulated valve, whereby the amount of gas being withdrawn from the suction object of the pump may be changed.
U.S. Pat. No. 4,329,201 (Albany International Corp.) relates to dewatering the web on the wire. According to said publication, e.g. a liquid ring pump or a centrifugal exhauster may be utilized for water removal. An essential feature of the idea described in the publication is the attempt of using constant vacuum for dewatering. According to the publication, this is achieved by employing in the dewatering system two water removal pipes, over which the wire passes. In the case of a new wire, only one water removal pipe is employed, but when the vacuum in said pipe increases a second pipe is opened and the suction from the vacuum means is applied to the wire also via the second pipe. The solution according to the publication utilizes also a control valve, which, together with the vacuum control, keeps the vacuum applied to the suction means constant. The publication further mentions, how the dwell time of the wire at each suction pipe may be increased, if necessary, by either increasing the width of the suction slot in the suction pipe in the moving direction of the wire or by decreasing the travelling speed of the wire.
Like the previous publication, U.S. Pat. No. 4,466,873 (Albany International Corp.) is also related to dewatering the fourdrinier section of a paper machine. The apparatus of this patent comprises an adjustable-speed vacuum pump, which may be either a conventional vacuum pump or a vacuum centrifugal pump, i.e., centrifugal exhauster. The purpose of the apparatus is to control the dewatering in two phases so that during the first phase the rotational speed of the pump is kept constant, whereby the pump creates a vacuum sufficient for dewatering the web. As the wire gradually clogs, the vacuum in the suction boxes increases. When the vacuum in the last suction box has reached a predetermined value, the control unit enforces the vacuum pump to decrease its rotational speed so that the vacuum remains in said maximum level thereafter.
The newest alternative for suction means in the market is a so-called high-speed vacuum pump, which is, in fact, a centrifugal pump applied for pumping gas. The use of said pump in dewatering in paper production is described in Fl patent 97244. Said publication concerns, e.g., a problem arising with the fabric, felt or wire, of the paper machine gradually tending to be clogged. In other words, the permeability of the fabric changes with the passing of service life. In said publication, the problem is solved so that when the amount of liquid to be removed by applying suction to the object, e.g. a suction box or a group of suction boxes, is known, the amount of liquid removed from said object is measured and compared with the predetermined target value and the operation of the suction system is adjusted accordingly. The easiest method of doing this is to adjust the rotational speed of the vacuum pump, because for example an increase in the rotational speed increases both the volume flow and the vacuum level of the vacuum pump. As a second alternative adjustment, the adjustment of both the rotational speed and the capacity of the vacuum pump, i.e. volume flow, is described.
A problem relating to the solution described in said publication is that the system comprises a mechanical device, flow meter, on which the controlling of the system is based. In addition to being a costly additional investment, said device is sensitive to damages, at least in long-term use, whereby the accuracy of the whole paper production or a corresponding process is endangered. Furthermore, the use of a flow meter, as such, brings no additional value for the system, but other interesting points, such as e.g. monitoring the condition of the suction object, such as the felt or the like, determining the need to change the felt etc. are still existing problems in the described prior art methods.
In the following, the use of a vacuum pump arrangement is described more precisely, as an example in connection with the vacuum system of a paper machine. The applications for the use of both various prior art vacuum pump systems and the method and vacuum pump arrangement of the present invention are, in principle, divided into two categories.
In the first category, suction is effected both to the fabric and the pulp web. In these cases, when the same type of pulp is being treated, the flow resistance remains almost constant regardless of the resistance of the felt, because the resistance of the pulp web is decisive. The flow resistance changes remarkably when the paper grade is changed. The conventional operating method is to determine a constant vacuum level for each paper grade, with the aim to monitor the level during the operation. Typical objects whereto vacuum is applied are, e.g., suction boxes of the web formation section of paper, paperboard and pulp drying machines, various suction rolls in the formation and press sections and various vacuum filters in the causticizing plant of a pulp mill and e.g. in the fertilizing and basic metal industry.
In the second category, vacuum is applied to the fabric only. In this case, the flow resistance increases with the aging of the fabric, due to fouling and compressing of the fabric. Typical suction objects are, e.g., the suction boxes of the felt-conditioning section of a paper machine.
With these suction boxes, placed at the press section of a paper machine, the fabric is dewatered by withdrawing air through it. The three basic functions of felt conditioning are:
1) water removal (part of the water in the fabric has been transferred from the paper web to the fabric on the press section),
2) cleaning (part of the water has come from cleaning jets used for cleaning the fabric) and
3) maintaining a level moisture profile in the fabric in the cross direction of the machine.
Traditionally, a vacuum pump has been used in connection with this kind of applications so that by the vacuum pump a constant-volume flow is withdrawn through the fabric, whereby the vacuum level increases with the aging of the fabric. A running method of this kind is achieved automatically by a displacement-type vacuum pump, e.g. a liquid ring pump.
However, the objective of felt conditioning is not to maximize water removal from the fabric, because an excessively dry fabric does not perform its function properly in the dewatering taking place in the press nip. The amount of air withdrawn through the fabric by means of a suction box is determined so that a suitable moisture ratio is ensured in the fabric. The moisture ratio is the relation of the weight of the water in the fabric to the weight of the fabric. Excessive air flow creates a too low moisture ratio and a too small air flow, accordingly, creates a too high moisture ratio. In practice it has been noticed, though, that said running method based on constant-volume flow does not lead to an optimal moisture ratio, but the moisture ratio varies even to a detrimental extent.
A solution of e.g. the problem mentioned above has shown to be a method of adjusting the vacuum level of felt conditioning by measuring the moisture of the fabric after the suction box and aiming to keep the moisture at a desired level by regulating the rotational speed and operating point of the vacuum pump.
A second possible method for solving e.g. said problem, is to measure the amount of water discharging from the suction box and the amount of spray waters and on that basis try to estimate the moisture of the felt and the vacuum needed. However, an additional problem arises with the estimating of the water amount displaced from the web into the felt in the press nip, which problem, nevertheless, is far from the category of moisture ratio variations of the running method based on constant-volume flow.
When trying to create an optimal running method for paper machines or the like in a case when the suction object comprises both fabric and pulp web, a prior art method is to remove water straight from the pulp by drawing air through the pulp and fabric. In a paper machine, the goal is to remove as much water from the pulp prior to the drying section as possible, because the cost of mechanical water removal (pressing and absorption) is only about a fourth of the cost of water removal carried out on the drying section. However, water removal has to be carried out gradually. For suction objects whereto suction is applied first and where the moisture of the web is the highest, an excessively high vacuum level may not be employed, as it would lead to excessive compression of the pulp web and reduce water removal in the subsequent stages. Conventionally, the aim has been to maintain the vacuum level constant in each suction object. The vacuum level chosen depends on the kind of pulp to be treated.
When using constant-speed liquid ring vacuum pumps, the vacuum level is regulated by means of a vacuum control valve with which a required amount of air is introduced to the vacuum system from outside. This kind of regulation method, spending a lot of energy, is very problematic, as part of the capacity of the vacuum pump is used to pump air from outside.
In the case of this prior art paper machine technology, vacuum systems based on the use of liquid ring pumps have generally been based on technology, according to which the vacuum created by the vacuum pump, or at least the amount of water removed by means of it, is regulated on the basis of experience. For instance, on the fourdrinier section of a paper machine, the vacuum level has traditionally been regulated according to the gloss of the pulp web. This has been practiced so that the operator of the paper machine knows from experience, on which stage of the fourdrinier section the pulp web should start to lose its gloss. The loosing of gloss means in practice that the water film on the surface of the pulp web is no longer uniform, but the amount of water in the web has been decreased to such an extent that almost all the water is absorbed into the web. When the felt of the paper machine starts to be clogged, the glazing part of the web increases, whereby an experienced operator knows that the vacuum level should be increased in order to absorb the desired amount of liquid from the web. However, this kind of regulation method is not the best possible, demanding accurate monitoring and regardless of that not providing the most accurate possible method of controlling the water removal. Neither does this monitoring method give a clear sign of the need to change the fabric of the paper machine, but the changing is also practiced based on experience, in most cases on the basis of the service life of the fabric. In other words, in practice a service life has been determined for each fabric, after which time the fabric is changed regardless of its actual condition.
By means of an adjustable-speed vacuum centrifugal pump, or a centrifugal exhauster, utilized in the vacuum system according to the invention, the vacuum level may be regulated by changing the rotational speed of the pump, whereby the capacity needed for creating the desired vacuum is optimized and power consumption is decreased, as the vacuum level is lower than in prior art solutions.
One of the most optimal running methods according to the invention in this kind of arrangement is to measure the moisture of the paper web after the press section prior to the drying section and try to maximize the dry solids content by regulating the vacuum in all suction objects at the same time. The appropriate vacuum level in each suction object and the relation of vacuums between various suction objects may be determined e.g. by means of an intelligent control system and fuzzy logic.
A second method of regulating the operation of water removal is to measure the discharging water amounts, for instance in a way described in Fl patent 97244, and regulate the vacuum levels accordingly. An essential point is that the aim is to maximize not the amount of water discharging from a single suction object but the total amount of water discharging from all suction objects.
A further essential feature of the vacuum arrangement according to the invention and the method applied in connection with it is that it may also be applied to monitoring the condition of the fabric. As already stated before, the permeability of the fabric changes in use both for clogging and compressing of the fabric. A sufficient change in permeability, possibly together with other factors, e.g. results of moisture measuring of the web and/or the fabric, signals the need to change the fabric.
As a second possibility for monitoring the condition of the fabric is trend monitoring (life-time control measurement) by utilizing the invention, which makes it possible to cormpare different fabrics and thus find the best possible fabric for each application. In other words, the invention makes it possible to compare e.g. between different paper fabrics from various producers or paper fabrics made of various materials as well as the applicability of different fabrics for different pulps.
Further, by utilizing the invention it is possible to monitor the operating condition of the pump, e.g. fouling, and signal it, based on data about the pump and its operation measured from the vacuum system and the performance chart of a new pump recorded in the system.
A new vacuum pump arrangement according to the invention, comprising a vacuum pump with adjustable rotational speed described e.g. in said Fl patent 97244, a suction object and a measuring and controlling system, eliminates e.g. the disadvantages of prior art methods described above and aims at optimizing water removal from various objects of application. A characteristic feature of the vacuum pump arrangement used in the invention is that it utilizes various information gathered about the operation of the pump, such as e.g. the rotational speed, power consumption and torque of the pump etc.
A characteristic feature of the arrangement according to the present invention applied e.g. in connection with a paper machine is that during the operation, the performance chart of the pump has been recorded into the control/central unit of the pump, e.g. in a form where both the rotational speed and the torque or power input of the pump are depicted in the vacuumxe2x80x94volume flow co-ordinate by groups of graphs, on basis of which co-ordinate the air flow being pumped is calculated based on measuring results (e.g. the rotational speed and torque or power input of the pump). On the basis of the air flow and measured vacuum level it is-then possible to calculate the permeability of the fabric on the suction box or the permeability of the fabric and the paper web together. This allows for monitoring changes in the density of the fabric or the paper web as a function of time and thus automatically carry out changes needed in the process or in the reconditioning of the fabric.
According to a preferred embodiment of the invention, the resistance function, i,e, the so-called resistance curve of the suction object is recorded in the memory of the control/central unit of the pump. The resistance function is preferably recorded in two different situations. Firstly, a resistance function f1, is recorded in the memory of the control/central unit which resistance function comprises the initial state where the felt fabric or filter material, through which the medium is withdrawn by means of the pump, is new and un-clogged. According to a second preferable embodiment of the invention, also a resistance curve, i.e. resistance function, f2 is recorded in the memory of the central unit, which resistance function comprises the same resistance components except that material, through which the vacuum is applied, such as the fabric or filter material, has been in use and is clogged. Said resistance functions are monitored during the operation of the vacuum pump system in relation to the operating point of the pump (rotational speed, vacuum level reached at the suction side at the rotational speed in question and the flow amount of pumped material reached at the corresponding vacuum level (e.g. m3/time unit). The operating point of the pump means the intersection of the resistance curve and the rotational speed curve. Thus, the aim is to control the pump so that its operating point is kept within the allowed area (between the graphs of the resistance functions) in the volume flowxe2x80x94vacuum co-ordinate. Another method of controlling the pump is that in accordance with the operating point e.g. the cleaning of the fabric of the paper machine is regulated (that is, e.g. when the resistance curve rises excessively, the use of cleaning sprays may be intensified or more cleaning sprays may be employed) and/or the rotational speed of the pump is regulated and thus, with clogging of the fabric, the rotational speed of the pump is accelerated.
According to a second preferable embodiment of the invention, the resistance functions fa, fb, fc of different types of felt may be recorded in the memory of the control/central unit 100 of the pump, and from the memory of the central unit of the pump said recorded functions fa, fb, fc may be utilized always when the type of felt is changed. Naturally, it is possible to record also the resistance function pairs, i.e. resistance functions of both new and clogged felt. Concerning various felts, it is also possible to practice long-term trend monitoring between different types of felt, on the basis of which trend monitoring the most suitable felt for each paper/paperboard pulp may be chosen.
The characteristic features of the control system for a vacuum pump and the method of regulating said pump according to the invention are described in the appended patent claims.
In the following, the invention is described with reference to present figures illustrating some preferred embodiments of the invention which, however, are not meant to restrict the invention.